Storage tanks which are being currently manufactured for underground storage purposes are double walled. An inner tank is a source of primary containment for the stored liquid. An outer wall serves as a secondary containment for the stored liquid in the event the inner tank develops a hole or crack which allows the liquid to escape.
A predominate number of the double walled storage tanks offered for sale are made from a fibrous reinforced resinous material and are commonly referred to in the industry as a FRP tank. Such tanks are light weight when compared to steel tanks. They usually have a cylindrical-shaped side wall and have domed end walls. Sufficient wall strength of the FRP tanks is achieved by the use of spaced support ribs extending circumferentially around the cylindrical-shaped side wall. The support ribs significantly increase the ability of the tank to withstand internal load forces from the stored liquid and external load forces from being buried underground.
It has been found that double walled storage tanks, while more reliable than single walled tanks, do occasionally fail. Any wall failure can cause extensive environmental damage if not detected early and promptly repaired. Leak detection means which are in communication with a space between the inner and outer walls are capable of detecting liquid and/or vapor, a change of pressure, a change of liquid level, or some other characteristic and then signaling that a potential problem exists. Any small hole or crack in an interior wall area normally can be detected and repaired prior to any significant liquid loss. Structural damage in this part of the tank normally starts small and stays small for a sufficient time to allow detection, removal of liquid and possible repair. However, a structural failure in the support rib area of the tank sometimes means a collapse of the tank is imminent. More radical remedial action may be required to prevent a possible total tank collapse and lost of several thousand gallons of stored liquid into the soil and natural aquifers of the area.
Known double wall storage tank systems typically do have leak detection means. However, any signal from the leak detection means merely indicates there is a structural failure somewhere in the tank system. The type or degree of failure is normally not known. U.S. Pat. No. 4,951,844 does disclose a double walled storage tank system wherein a flat end wall area is separately monitored from the side wall area of the tank. The flat end wall area is a vulnerable area of many tanks and added strength in the area coupled with independent monitoring of it is of benefit. However, the support rib areas of a tank are also a critical area where a structural failure can be costly.
In accord with a need for an improved tank design with greater capability of detecting specific structural damage, there has now been developed a double walled ribbed storage tank system design having the capability of independent monitoring of support rib areas and secondary containment areas. The tank system solves a problem which has the potential for significant environmental consequences in a very economical and reliable manner.